This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application entitled OPTICAL PREAMPLIFIER HAVING ONE STAGE CONFIGURATION earlier filed with the Korean Industrial Property Office on 28, Nov. 2001 and there duly assigned Serial No. 74699/2001.
1. Field of the Invention
The present invention relates to an optical transmission system for a long distance, and more particularly, to an optical preamplifier including one amplification unit for outputting an optimum level of an optical signal to an optical receiver.
2. Description of the Related Art
Generally, as shown in FIG. 1, an optical transmission system includes an optical transmitter for generating an optical signal, an optical power amplifier OPA for amplifying powers of the optical signal, multiple optical line amplifiers OLAs according to an optical signal damping, an optical preamplifier OPRA for compensating damping amounts of the optical signal, and an optical receiver. The optical signal from the optical transmitter is transmitted to the optical receiver through OPA, OLAs, and OPRA.
The wavelength of the optical signal differs according to the optical transmission system. For the optical transmission system for a long distance, the wavelength of 1550 nm (nanometers) is typically used because it belongs to a bandwidth of a gain wavelength of the optical amplifier and the loss of the wavelength is small during a transmission. On the contrary, for the optical transmission system for a short or medium distance, the wavelength such as 1300 nm is used because the dispersion value of the wavelength is nearly zero even though the wavelength loss is large.
In other words, the wavelength of the optical channel belongs to a bandwidth of 1550 nm in an optical transmission system of high speed such as more than 10 Gbps (gigabits per second). As described in the above statements, such an optical channel of a bandwidth of 1550 nm has a dispersion value making some troubles in a long distance transmission of the optical signal. Therefore, to compensate the dispersion value of the optical fiber, a dispersion compensation fiber module DCFM is normally used in the OLA and the OPRA.
The DCFM compensates dispersions of the wavelength while transmitting an optical signal. However, when the optical intensity of the inputted optical signal is more than 0 dBm (decibels) because the damping amount, such as 8 dBm, is large and the diameter of the core is too small, a signal distortion can happen due to a non-linear phenomena. To solve such problems, the conventional optical transmission system for a high speed and a long distance includes an optical preamplifier with two amplification units and a DCFM between the two amplification units. The amplification unit is normally an erbium doped fiber EDF.
The conventional optical preamplifier including two amplification units, EDFs, is illustrated in FIG. 2. Generally, the conventional optical preamplifier applies two optical pumping methods to get gains. One method is that both ends of the DCFM respectively include a laser diode LD and a gain media such as an EDF to get gains in both ends of the DCFM independently. The other method is to use a wavelength division multiplexor WDM to get gains, a first gain and a second gain. The first gain is acquired by the way of dividing the output of a pumping LD by a first gain media EDF, and sequentially, the second gain is acquired by the way of dividing the output of a pumping LD by a second gain media EDF. The conventional optical preamplifier of FIG. 2 illustrates the latter pumping method.
Referring to FIG. 2, the conventional optical preamplifier includes a first erbium doped fiber EDF121, a DCFM, and a second erbium doped fiber EDF222. The EDF121 amplifies an input level of the optical signal, such as xe2x88x9210xcx9cxe2x88x9220 dBm, to an out level of about xe2x88x923xcx9cxe2x88x926 dBm. The EDF222 compensates the losses of the optical signal generated while the optical signal from the EDF121 is passing through the DCFM, and amplifies the optical signal to maintain the entire optical outputs in xe2x88x923 dBm. The amplification procedures are illustrated as follows.
The EDF121 transmits the optical signal input from the input terminal IN to a first wavelength division multiplexor WDM1 while the first wavelength division multiplexor WDM1 inverse-multiplexes the optical signal to a management channel of 1510 nm and a signal channel of 1550 nm. Parts of the optical signal of 1550 nm inverse-multiplexed in the WDM1 are divided through a first tap coupler TAP1, and a photo detector LOS monitors optical losses during this process. The optical signal output from the TAP1 sequentially passes through a first optical isolator ISO1, and is received to one input terminal of a second wavelength division multiplexor WDM2. The other input terminal of the WDM2 receives a pumping light of 980 nm to multiplex the optical signal.
The optical signal multiplexed in the WDM2 is amplified through the EDF121, and a third wavelength division multiplexor WDM3 inverse-multiplexes the optical signal of 1550 nm and the pumping light of 980 nm. The optical signal of 1550 nm passes through an optical isolator band pass filter ISOF, and is received to the DCFM. The DCFM compensates the dispersion value included in the optical signal.
The EDF222 transmits the optical signal of 1550 nm compensated in the DCFM to a second optical isolator ISO2 and then to one input terminal of a fourth wavelength division multiplexor WDM4. The other input terminal of the WDM4 receives a pumping light of 980 nm inverse-multiplexed in the WDM3, and the WDM4 multiplexes the optical signal of 1550 nm and the pumping light of 980 nm. The multiplexed optical signal passes through the EDF2, and the EDF2 amplifies the optical signal of 1550 nm. The amplified optical signal of 1550 nm passes through a third optical isolator ISO3, and is transmitted into a manual turnable filter MTF. The MTF eliminates maximum amounts of an amplified spontaneous emission ASE generated during the process of amplifying the optical signal. Finally, the optical signal is transmitted into a second tap coupler TAP2. The TAP2 divides a portion of the optical signal for use in an auto power control APC circuit and outputs the other optical signal to the optical receiver through the output terminal OUT. The APC circuit adjusts an optical intensity outputted from the pumping LD to maintain the entire optical output regularly even though the optical intensity of the inputted optical signal is variable.
The input terminal of the above described conventional optical preamplifier receives an optical signal of xe2x88x9210xcx9cxe2x88x9226 dBm, and the output terminal outputs an optical signal of xe2x88x923 dBm to the optical receiver. However, the optimum intensity of the optical signal input to the optical receiver is xe2x88x926xcx9cxe2x88x928 dBm. Therefore, an additional variable or fixed damper is necessary between the optical preamplifier and the optical receiver to adjust the output level of the optical signal from xe2x88x923 dBm to xe2x88x926xcx9cxe2x88x928 dBm.
In other words, because the conventional optical preamplifier includes two amplification units, the configuration is complex and the production cost is increased. Furthermore, because the optical intensity of the optical signal from the optical preamplifier is higher than the optimum intensity of the optical signal to the optical receiver, an additional device is necessary.
It is therefore, an object of the present invention to provide an optical preamplifier with one amplification unit for amplifying an optical signal, compensating dispersion of the optical signal after receiving the optical signal from an optical transmitter and supplying the optical signal to an optical receiver.
It is another object to provide an optical preamplifier that is simpler to manufacture in order to reduce production costs without degrading the performance of the optical preamplifier.
It is yet another object to provide an optical preamplifier that can be operated in optimum modes without additional devices.
These and other objects may be achieved by the preferred embodiments of the present invention providing an optical preamplifier with one amplification unit for amplifying an optical signal, compensating dispersion of the optical signal after receiving the optical signal from an optical transmitter and supplying the optical signal to an optical receiver, as the optical preamplifier includes a first wavelength division multiplexor WDM1 for multiplexing the optical signal and a pumping light; an erbium doped fiber EDF for amplifying the optical signal multiplexed in the WDM1; an optical isolator band pass filter ISOF for passing the optical signal with a bandwidth of a predetermined wavelength only among the optical signal amplified by EDF; a DCFM for damping an optical intensity of the optical signal within an optimum level of the optical signal for the optical receiver, while compensating the dispersion of the optical signal from the ISOF; and a first tap coupler for dividing the optical signal output from the DCFM in constant ratio, and for outputting the optical signal to a circuit to control the optical intensity of the pumping light and to the optical receiver, wherein the optical signal in the EDF is amplified in a range of not generating a non-linear phenomena in the DCFM.
Furthermore, the present invention further includes a second tap coupler, installed in front of the DCFM, for dividing a portion of the optical signal; a photo detector LOS for detecting an optical intensity of the optical signal by using the optical signal divided in the second tap coupler; and an optical isolator ISO, installed between the second tap coupler and the DCFM, for interrupting a retrogression of the optical signal.
Further, the predetermined wavelength is 1550 nm.